Project topics and materials
Get Complete Project Material File(s) Now! »
WATER FOOTPRINT NETWORK METHODOLOGY CALCULATIONS
For this chapter the goal was set to account blue, green and grey WFs of the main crops grown on the Steenkoppies Aquifer according to the WFN method. A sustainability assessment of these WFs has been done in Chapter 6. The scope defined in Section 3.2 applies to these WFs. Using the verified modelled data and long term simulations from 2004 to 2013, blue and green WFs were calculated according to the WFN methodology (Hoekstra et al., 2011) as given in Equation 2-3 and Equation 2-5, respectively (Chapter 2). Water footprints were affected by a lack of solar radiation data, and could therefore not be calculated for the years before 2004. This issue is further discussed in Chapter 5. In SWB the result under irrigation applied has taken effective rainfall into account and therefore represents the irrigation requirement. As per Hoekstra et al. (2011), yield in fresh mass was used. Water footprints were also calculated using yield in dry matter as an alternative.
The general standards for N in wastewater of 15 mg ℓ-1 (Department of Water Affairs and Forestry, 1999) was taken as Cmax. This value was taken, because it is a standard given for wastewater from industries, as it is assumed that the water would be diluted further downstream. This value would result in a lower grey WF, compared to the WF that would result if environmental standards are used for Cmax, and therefore it would represent the lowest grey WF that could be obtained. If this lowest grey WF would suggest some impacts on water quality, one would expect to see this impact on the water quality in the Steenkoppies Aquifer. The natural concentration (Cnat) is the N concentration of the water if no human influences are present. Despite intensive agricultural activities on the Steenkoppies Aquifer, the water in the aquifer has very low N concentrations, with an average of 0.3 mg ℓ-1 (Department of Water Affairs, 2014). Thus, the low average natural N concentration of the aquifer was considered to represent natural concentrations and was taken as Cnat. Chapter 5.3.4 further discusses the observation that the aquifer does not yet reflect the expected impacts of intensive agricultural activities.
The N load that leaches into the aquifer was determined by estimating the surplus N applied to the crops together with a leaching-runoff factor, according to the method provided by Franke et al. (2013). To determine the surplus N, the N content of the harvested product (which represents the portion of N that is taken up by the plant and removed from the field) was subtracted from the N application per crop. Typical N fertiliser application rates for carrots, cabbage, beetroot and lettuce were provided by farmers on the Steenkoppies Aquifer, and N application to broccoli was assumed to be the same as for cabbage. Nitrogen application given by the Fertiliser Society of South Africa (Misstofvereniging van Suid Afrika, 2007) was used for beetroot, maize and wheat. For maize and wheat, the application rates were also linked to expected irrigated yields for the aquifer. The N contents of the crops were taken from the literature (Alexandrova and Donov, 2003, ANZECC and ARMCANZ, 2000, Mossé et al., 1985, Petek et al., 2012, Sorensen, 1998).
HYDROLOGICAL WATER FOOTPRINT METHODOLOGY CALCULATION
The verified SWB model estimates also provided the data used to calculate WFs according to the hydrological methodology. The hydrological methodology has not proposed a water quality impact metric, and uses the grey WF methodology proposed by the WFN. Blue WFs are based on the change in groundwater storage and is calculated as per Equation 2-17 (Chapter 2) (Deurer et al., 2011). In the original study Deurer et al. (2011) assumed that all runoff became drainage, because of the flat topography of their study area. This is why runoff in this formula reduces the blue WF on the aquifer. For this study runoff was also assumed to be zero, due to the absence of surface runoff on the Steenkoppies Aquifer. Rainfed conditions cannot be modelled for the vegetables on the Steenkoppies Aquifer, because some crops fail due to low rainfall conditions in winter. In SWB this is reflected by extremely low ET values and underdevelopment of the harvestable crop. Thus, for blue WF calculations total drainage under irrigated conditions was used instead of Dr plus Dir. This however presented a problem with calculating green WFs, which is based on the change in soil moisture originating from rainfall Equation 2-18 (Chapter 2), where ET under rainfed conditions are required (Deurer et al., 2011). For this reason, the green WF was assumed to be zero, because over the long-term green water will be replenished by rainfall and the changes in soil water storage would be negligible.
The hydrological methodology, which considers the water balance over an entire calendar year, is not compatible with estimating the WFs of a single short season vegetable crop such as those cultivated on the Steenkoppies Aquifer. Therefore, the annual WF was calculated for typical cropping sequences within a twelve-month period. Fresh weight then equals the combined weight of all crops produced in the sequence. The WF will thus represent a combination of crops, instead of one single crop. A crop rotation of carrots and cabbage is typical on the Steenkoppies Aquifer (Table 3-1). A two-crop sequence of winter cabbage planted on 1 May each year and summer carrots planted on 7 November each year was therefore selected. Due to the intensive farming activities on the aquifer, a three-crop sequence was also selected, with winter broccoli planted on 1 May each year, spring cabbage planted on 25 August each year, and summer beetroot planted on 13 December each year. The crops selected for the three-crop sequence was based on the length of the growing seasons, so that the sequence can be completed in one calendar year for comparison with WFN results. Broccoli, which had a high WF according to the WFN results, was specifically included for comparison with WFN results.
In order to compare the hydrological WF results of the two-crop sequence with the WFN results the average between WFN WFs of carrots planted in summer and cabbage planted in winter was taken. Likewise, the average WFs according to the WFN for winter broccoli, spring cabbage and summer beetroot was taken to compare the hydrological WFs results of the three-crop sequence.
LCA WATER FOOTPRINT METHODOLOGY CALCULATIONS
A WS Index of 0.78, calculated by Pfister et al. (2009) for the area in which the Steenkoppies Aquifer is located, was used to convert the WFs of the crops on the Steenkoppies Aquifer. The blue WFs according to the WFN methodology were used to calculate LCA WFs, because these WFs quantify the volume of blue water used to produce a product. Site specific WS Indices for the Steenkoppies Aquifer were also calculated for five distinct periods classified in terms of the intensity of irrigated agriculture (Chapter 6) according to the methodology proposed by Pfister et al. (2009). The withdrawal to availability ratio (WTA) for regulated catchments were calculated according to Equation 2-11 (Chapter 2) given by Pfister et al. (2009) The catchment scale agricultural blue WFs estimated in Chapter 6 for the five periods were taken as the WU and average outflows from the Maloney’s Eye from 1909 to 1950 were taken as WA, because abstractions for irrigated agriculture only commenced after this period, and this average was assumed to represent natural outflows. Long term monthly and annual precipitation data from 1950 to 2012 was used to calculate the VF according to the formula given by Pfister et al. (2009) (Equation 2-13 of Chapter 2). The WS Indices for each of the five periods were compared to determine if it produces a relatively constant result that can be applied to a catchment over the long term. The WS Indices that were calculated for the five periods were also compared to the WS Index of 0.78 calculated for the region by Pfister et al.
Chapter 1: Introduction and Background
1.1 Introduction
1.2 Hypotheses, Aims and Objectives of this Study
1.3 The Steenkoppies Aquifer
1.4 Overview of the Chapters
Chapter 2: Literature Review
2.1 Introduction
2.2 Estimation of Water Footprints According to Different Methodologies
2.3 A Review of Published Comparisons Between Different Methods
2.4 Water Footprints: Potential Shortcomings and Key Challenges
2.5 Discussion and Recommendations
2.6 Conclusion
Chapter 3: Evaluation of methodologies to estimate water footprint of producing selected vegetables in the Steenkoppies Aquifer
3.1 Introduction
3.2 Materials and Methods
3.3 Results
3.4 Discussion
3.5 Conclusions
Chapter 4: water footprints of crops in the packhouse
4.1 Introduction
4.2 Materials and Methods
4.3 Results
4.4 Discussion
4.5 Conclusion
CHAPTER 5: Understanding complexities in estimating water footprints of vegetable crops
5.1 Introduction
5.2 Materials and Methods
5.3 Results
5.4 Discussion
5.5 Conclusions
Chapter 6: Catchment scale Water Footprint of the Steenkoppies Aquifer
6.1 Introduction
6.2 Materials and Methods
6.3 Results
6.4 Discussion
6.5 Conclusion
Chapter 7: Water footprints of vegetable crop wastage produced on the Steenkoppies Aquifer
7.1 Introduction
7.2 Materials and Methods
7.3 Results
7.4 Discussion
7.5 Conclusion
Chapter 8: Estimating the water footprint of fancy lettuce (Lactuca sativa) cultivars cos and butterhead
8.1 Introduction
8.2 Materials and Methods
8.3 Results
8.4 Discussion
8.5 Conclusion
Chapter 9: Discussion
9.1 Comparison Between Water Footprint Methods
9.2 Packhouse Water Footprints
9.3 Complexities Involved in Calculating Water Footprints
9.4 Catchment Scale Water Footprints
9.5 Water Footprints of Wastage
9.6 Water Footprints of Fancy Lettuce Cultivars
Chapter 10: Conclusion
10.1 Value of Water Footprint Network Water Footprints on a Local Level
10.2 Value of Water Footprint Network Water Footprints on a Regional Level
10.3 Use of Water Footprints on a National Level
List of References
